Oakleaf, you're right about terrain, care of chain etc... After reading how people here on this particular subject are taking care of their chains, that 1500 miles is based on people such as these. Now that number is a rough estimate, but it gives people the idea. Some will get more miles and some will not, but that's to be expected. Obviously if your cassette happens to have a bit more wear, then the next chain will also have a shorter life.

For my personal bikes, I have been using this formula for quite some time, and it seems to do a pretty good job. If I'm paying $50-$60 for a chain, and $300 or so for a cassette(Campy) then it's pretty easy to do the math. A good portion of my customers are doing the same thing, but as you said.. it depends on conditions and the care that someone gives that chain.